Hubbard U library and high throughput exploration of spin Hamiltonian parameters for the rational design of metal trihalides MX3_3 (M={Ti,V,Cr,Fe}, X={Cl,Br,I}) with high Curie temperature

Metal trihalides (MX$_3$) are the most important family of 2D magnetic materials, being the chromium trihalides the most studied 2D magnets. The discovery of CrI$_3$, the recent obtention of CrCl$_3$ in-plane magnetic monolayer and the role of CrBr$_3$ inducing topological superconductivity in NbSe$_2$, may serve to showcase the active research being done in this family nowadays. Despite the high impact of these materials, most of the members in the MX$_3$ family are still unexplored and constitute an untapped source of interesting physical properties. Stimulated by the most recent advances in straintronics and aware of the crucial role of the dielectric screening, we present here a high throughput methodology to automatize the exploration of 2D materials. Employing this methodology, we studied the MX$_3$ family (M= Cr, Fe, V, Ti; X= Cl, Br, I) with the goal of advancing towards the solution of the most problematic issue in these materials, namely the Curie temperature. We use a particular case to show how this methodology allows us to obtain a complete description of the magnetic interaction picture (J$_{iso}$, J$_{xx}$, J$_{yy}$, J$_{zz}$) up to third neighbors condensed in a single effective equation per parameter, describing magnetic interaction in terms of the strain and the Hubbard U parameter. Additionally, and because of the important role of the Hubbard U in MX$_3$ materials, we provide a library of self-consistent calculated Hubbard U for the principal pseudopotential families. The work presented herein advances in the description of the still unexplored MX$_3$ materials, opening the door to a rational design of 2D magnetic materials

Magnon straintronics in the 2D van der Waals ferromagnet CrSBr from first-principles

The recent isolation of two-dimensional (2D) magnets offers tantalizing opportunities for spintronics and magnonics at the limit of miniaturization. One of the key advantages of atomically-thin materials is their outstanding deformation capacity, which provides an exciting avenue to control their properties by strain engineering. Herein, we investigate the magnetic properties, magnon dispersion and spin dynamics of the air-stable 2D magnetic semiconductor CrSBr ($T_C$ = 146 K) under mechanical strain using first-principles calculations. Our results provide a deep microscopic analysis of the competing interactions that stabilize the long-range ferromagnetic order in the monolayer. We showcase that the magnon dynamics of CrSBr can be modified selectively along the two main crystallographic directions as a function of applied strain, probing the potential of this quasi-1D electronic system for magnon straintronics applications. Moreover, we predict a strain-driven enhancement of $T_C$ considering environmental screening by ~30%, allowing the propagation of spin waves at higher temperatures

Ultra-broad spectral photo-response in FePS3 air-stable devices

Van der Waals materials with narrow energy gaps and efficient response over a broadband optical spectral range are key to widen the energy window of nanoscale optoelectronic devices. Here, we characterize FePS3 as an appealing narrow-gap p-type semiconductor with an efficient broadband photo-response, a high refractive index, and a remarkable resilience against air and light exposure. To enable fast prototyping, we provide a straightforward guideline to determine the thickness of few-layered FePS3 nanosheets extracted from the optical transmission characteristics of several flakes. The analysis of the electrical photo-response of FePS3 devices as a function of the excitation energy confirms a narrow gap suitable for near IR detection (1.23 eV) and, more interestingly, reveals a broad spectral responsivity up to the ultraviolet region. The experimental estimate for the gap energy is corroborated by ab-initio calculations. An analysis of photocurrent as a function of gate voltage and incident power reveals a photo-response dominated by photogating effects. Finally, aging studies of FePS3 nanosheets under ambient conditions show a limited reactivity of the outermost layers of flakes in long exposures to air

Photoluminescence Enhancement by Band Alignment Engineering in MoS2/FePS3 van der Waals Heterostructures

Single-layer semiconducting transition metal dichalcogenides (2H-TMDs) display robust excitonic photoluminescence emission, which can be improved by controlled changes to the environment and the chemical potential of the material. However, a drastic emission quench has been generally observed when TMDs are stacked in van der Waals heterostructures, which often favor the nonradiative recombination of photocarriers. Herein, we achieve an enhancement of the photoluminescence of single-layer MoS2 on top of van der Waals FePS3. The optimal energy band alignment of this heterostructure preserves light emission of MoS2 against nonradiative interlayer recombination processes and favors the charge transfer from MoS2, an n-type semiconductor, to FePS3, a p-type narrow-gap semiconductor. The strong depletion of carriers in the MoS2 layer is evidenced by a dramatic increase in the spectral weight of neutral excitons, which is strongly modulated by the thickness of the FePS3 underneath, leading to the increase of photoluminescence intensity. The present results demonstrate the potential for the rational design of van der Waals heterostructures with advanced optoelectronic properties.The authors acknowledge funding from Generalitat Valenciana through grants IDIFEDER/2020/005, IDIFEDER/2018/061, PROMETEO Program and PO FEDER Program, the APOSTD/2020/249 fellowship for M.R., and support from the Plan Gen-T of Excellence for J.J.B. (CDEIGENT/ 2019/022), J.C.-F. (CIDEGENT/2018/005), and M.R.C (CideGenT2018004); from the Spanish MCINN through grants PLASTOP PID2020-119124RB-I00, 2D-HETEROS PID2020-117152RB-100, and Excellence Unit “María de Maeztu” CEX2019-000919-M; and from the European Union (ERC-2021-StG-101042680 2D-SMARTiES and ERC AdG Mol-2D 788222)

Modelling and tailoring 2D magnetism in van der Waals materials from first principles

The emergence of two-dimensional (2D) materials has revolutionised the landscape of condensed matter physics and material science. 2D materials exhibit extraordinary electronic and mechanical characteristics, offering unprecedented opportunities for exploring novel phenomena and engineering advanced functionalities in the limit of miniaturization. This thesis aims to contribute to expand the frontiers of knowledge in two main directions, (1) the rationalization of 2D magnetism and related phenomena from first principles and (2) the tailoring of magnetic properties in the limits of dimensionality. For this, first principles methods are used to simulate the most important 2D magnets in the field, namely FePS3 and the family of MPS3, CrI3 and the family of CrX3 and other new promising materials such as the CrSBr monolayer. This thesis aims to present an analysis of 2D magnetism from a fundamental perspective, that covers from its first origin in the formation of the electronic structure to the comprehension of the exchange mechanisms. The manifestation of magnetism in the atomic structure has been studied to understand magnetostriction in MPS3 and to underline the role of exchange interactions in the properties of 2D magnetic materials. Moreover, the distortions induced by magnetism in the crystal structure are employed to measure the magnetic order, providing a methodology to detect the still elusive antiferromagnetism. This thesis explores the possibility to tailor magnetic interaction via the design of new 2D materials, such as the Janus materials MPS3. The results presented exhibit the valuable possibilities offered by the Janus technique to induce SOC and symmetry breaking in 2D materials, affecting the magnetic anisotropy and DM interactions and thus, motivating the future exploration and synthesis of Janus 2D materials. Moreover, magnetic exchange interactions in 2D magnetic materials are rationalised from a different perspective based on the intrinsic competition of exchange interactions. In this extended perspective, individual exchange interactions can be seen as a complex magnetic system, where ferromagnetism and antiferromagnetism intrinsically coexist, forming different orbital pathways of interaction that mediate their intrinsic competition. The comprehension of 2D magnetism provided by this perspective is oriented in the last part of the thesis to practical applications, where the intrinsic competition of interactions in 2D magnets serves as a guideline to tailor 2D magnetism via external stimuli. The last chapters of this thesis illustrate how the intrinsic mechanisms can be rationally manipulated via the modification of the environmental screening or the distortion of the structures. As a proof of concept, this thesis introduces the concept of magnon straintronics in a 2D van der Waals magnet, that showcases the control of magnons manipulating the intrinsic competitions of exchange interactions in CrSBr and CrX3 using strain

Valence band electronic structure of the van der Waals antiferromagnet FePS3

Antiferromagnetic van der Waals materials have gained a lot of interest in recent years. They can be exfoliated down to the two-dimensional (2D) limit while potentially preserving intriguing properties of antiferromagnets, such as insensitivity to external magnetic fields and ultrafast spin dynamics in the THz range. The investigation of the electronic band structure of these materials is crucial to understand their behavior and thus to identify paths for future applications. Here, we investigate the valence band structure of one of the most studied 2D antiferromagnets -iron phosphorus trisulfide (FePS3)- using angle-resolved photoemission spectroscopy (ARPES) and compare our results with first-principles calculations based on Hubbard-corrected density functional theory (DFT+U). This allows us to identify the bands originating respectively from the Fe 3d, the S 3p, and the P 3p orbitals and to describe their dispersion throughout the whole Brillouin zone. Our results represent an important step towards an accurate theoretical description of the electronic properties of transition metal phosphorus trisulfides, which is a pre-requisite for understanding the behavior of antiferromagnetic materials at the 2D limit

Magnetic order in 2D antiferromagnets revealed by spontaneous anisotropic magnetostriction

The temperature dependent order parameter provides important information on the nature of magnetism. Using traditional methods to study this parameter in two-dimensional (2D) magnets remains difficult, however, particularly for insulating antiferromagnetic (AF) compounds. Here, we show that its temperature dependence in AF MPS3 (M(II) = Fe, Co, Ni) can be probed via the anisotropy in the resonance frequency of rectangular membranes, mediated by a combination of anisotropic magnetostriction and spontaneous staggered magnetization. Density functional calculations followed by a derived orbital-resolved magnetic exchange analysis confirm and unravel the microscopic origin of this magnetization-induced anisotropic strain. We further show that the temperature and thickness dependent order parameter allows to deduce the material’s critical exponents characterising magnetic order. Nanomechanical sensing of magnetic order thus provides a future platform to investigate 2D magnetism down to the single-layer limit.QN/van der Zant LabDynamics of Micro and Nano SystemsQN/Steeneken LabQN/vanderSarlabQN/Blanter Grou

Magnetic order in 2D antiferromagnets disclosed by spontaneous anisotropic magnetostriction

The temperature dependent order parameter provides important information on the nature of magnetism. Using traditional methods to study this parameter in two-dimensional (2D) magnets remains difficult, however, particularly for insulating antiferromagnetic (AF) compounds. We show that its temperature dependence in AF MPS3 (M(II) = Fe, Co, Ni) can be probed via the anisotropy in the resonance frequency of rectangular membranes, mediated by a combination of anisotropic magnetostriction and spontaneous staggered magnetization. Density functional calculations followed by a derived orbital-resolved magnetic exchange analysis confirm and unravel the microscopic origin of this magnetization inducing anistropic strain. We further show that the temperature and thickness dependent order parameter allows to deduce the material's critical exponents characterising magnetic order. Nanomechanical sensing of magnetic order thus provides a future platform to investigate 2D magnetism down to the single-layer limit.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.QN/van der Zant LabDynamics of Micro and Nano SystemsQN/Steeneken LabQN/Blanter GroupPrecision and Microsystems Engineerin

Magnetic order in 2D antiferromagnets revealed by spontaneous anisotropic magnetostriction

Abstract The temperature dependent order parameter provides important information on the nature of magnetism. Using traditional methods to study this parameter in two-dimensional (2D) magnets remains difficult, however, particularly for insulating antiferromagnetic (AF) compounds. Here, we show that its temperature dependence in AF MPS3 (M(II) = Fe, Co, Ni) can be probed via the anisotropy in the resonance frequency of rectangular membranes, mediated by a combination of anisotropic magnetostriction and spontaneous staggered magnetization. Density functional calculations followed by a derived orbital-resolved magnetic exchange analysis confirm and unravel the microscopic origin of this magnetization-induced anisotropic strain. We further show that the temperature and thickness dependent order parameter allows to deduce the material’s critical exponents characterising magnetic order. Nanomechanical sensing of magnetic order thus provides a future platform to investigate 2D magnetism down to the single-layer limit